WIND INSTRUMENTS WINDMILLS. 



71 



perceptible on the coasts of the Mediterranean, and 

 sometimes even as far north as Norway. We thus 

 perceive that, within the limits of from twenty- 

 eight to thirty degrees on each side of the equator, 

 the movements of the atmosphere are carried on 

 with great regularity ; but, beyond these limits, 

 the winds are extremely variable and uncertain, and 

 the observations made have not yet led to any sa- 

 tisfactory theory by which to explain them. It 

 appears, however, that, beyond the region of the 

 trade-winds, the most frequent movements of the 

 atmosphere are from the south-west in the north 

 temperate zone, and from the north-west in the 

 south temperate zone. This remark must be limit- 

 ed to winds blowing over the ocean, and in mari- 

 time countries ; because those in the interior of 

 continents are influenced by a variety of circum- 

 stances, among which the height and position of 

 chains of mountains are not the least important. 

 These south-west and north-west winds of the 

 temperate zones are most probably occasioned in 

 the following manner : In the torrid zone, there is 

 a continual ascent of air, which, after rising, must 

 spread itself to the north and south in an opposite 

 direction to the trade-winds below. These upper 

 currents becoming cooled above, at last descend 

 and mix themselves with the lower air : part of 

 them may perhaps fall again into the trade-winds ; 

 and the remainder, pursuing its course towards the 

 poles, occasion the north-west and south-west 

 winds of which we have been speaking. It. has 

 also been conjectured that these winds may fre- 

 quently be caused by a decomposition of the atmos- 

 phere towards the poles, from part of the air be- 

 ing at times converted into water. See Hurricane, 

 Whirlwinds, Harmattan, Simoom, &c. 



The following facts, illustrative of the course of 

 the winds in the North Atlantic, are of practical 

 interest. They are taken from a statement of pas- 

 sages made from 1818 to 1827, embracing a period 

 of ten years, and comprising 188 complete voyages. 



The passages from New York to Liverpool, during 

 this period, averaged each 24 days. 



Those from Liverpool to New York, 38 .. 



Shortest passage from New York to Liverpool, in 

 December 16 .. 



Longest, in December, 37 



Shortest passages from Liverpool to New York, 

 April and February, 22 



Longest passage, December to February, 71 .. 



These passages are reckoned from city to city. 



See Romney's Tableau des Vents, &c. (Paris, 1806, 

 2 vols.). 



WIND INSTRUMENTS. See Instruments. 



WINDMILLS. Pomponius Sabinus or Laetus, 

 a writer of the fifteenth century, says that wind- 

 mills were in use among the Romans ; but the si- 

 lence of Vitruvius and Seneca, who have spoken of 

 the advantages of wind, have led many writers to 

 doubt the truth of this statement. Some authors 

 have maintained that they were used in France in 



the sixth century, while others are of opinion that 

 they were brought into Europe by the crusaders ; 

 and Gibbon (ch. 61) says that they were first in- 

 vented in the dry country of Asia Minor. It is 

 certain that they were in use in the western coun- 

 tries of Europe in the twelfth century. (See Beck- 

 raann's History of Inventions, vol. i.) When wind 

 is employed as the first mover of machinery, it may 

 be applied in two ways 1. by receiving it upon 

 sails which are nearly vertical, and which give mo- 

 tion to an axis nearly horizontal, in which case the 

 machine is called a vertical windmill, because the 

 sails move in a vertical plane ; and, 2. by receiving 

 it upon vertical sails which move in a horizontal 

 plane, and give motion to a vertical axis, in which 

 case it is called a horizontal windmill. As a hori- 

 zontal windmill consists of vertical sails moving 

 horizontically around a vertical arbour or wind- 

 shaft, no motion would arise on exposing it to the 

 action of the wind, as the effect of the wind upon 

 the sails on one side would be counterbalanced by 

 its action upon the corresponding sails on the op- 

 posite side. Hence it is necessary either to screen 

 the sails on one side from the action of the wind, 

 or to construct the sails in such a manner that, 

 when they return against the wind, they present 

 only their edge to its action. The method of 

 screening the returning sails from the wind is adopt- 

 j ed in Tartary and some provinces of Spain, and is 

 the most simple that has been tried. When the 

 screen is not used, the sails may be fixed like float- 

 boards, with hinges, on the circumference of a large 

 drum or cylinder, so that, when they are to receive 

 the action of the wind, they stand at right angles 

 to the drum, and when they return against the 

 wind they fold down upon its circumference. Other 

 ingenious methods have also been devised for bring- 

 ing back the sails against the wind. In the verti- 

 cal windmill, on the other hand, the arms which 

 carry the sails revolve in a plane facing the wind. 

 In this arrangement, if the sails were in the same 

 plane with the arms, the wind would fall perpendi- 

 cularly upon them, and merely press the arms 

 against the building, perpendicular to the plane in 

 which they are designed to move. If, on the other 

 hand, the sails were perpendicular to the plane in 

 which the arms move, their edges would be pre- 

 sented to the wind, and would, therefore, offer no 

 resistance, and there would be no motion. In order 

 to make the arms revolve, the sails must, therefore, 

 be placed in some direction intermediate between 

 those of the wind and the plane in which the arms 

 revolve. In determining the angle at which the 

 planes of the sails should be inclined to the axis of 

 motion, or the direction of the wind, it is necessary 

 to consider the sail in motion ; and the neglect of 

 this element in the calculation has led to very great 

 errors in theoretical calculations. The sail being 

 in motion, the velocities of the sail and the wind 

 must both be taken into account ; for, if the sail 

 moved before the wind with a speed equal to that 

 of the wind itself, no effect would be produced. 

 The effect will depend on the difference of the 

 velocities, that being the velocity with which the 

 wind strikes the sail. Now, as the obliquity of 

 the sail to the wind should depend on the force 

 with which the wind acts upon it, and as those 

 parts of the sail which are nearer to the centre of 

 motion move more slowly than those which are 

 more remote, it follows that the position of the 

 sail should vary at different distances from the 

 centre of rotation. From the experiments of Mr 



